Spring入门第六课

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XML配置里的Bean自动装配

Spring IOC容器可以自动装配Bean。需要做的仅仅是在<bean>的autowire属性里指定自动装配的模式

ByType（根据类型自动装配）：若IOC容器中有多个与目标Bean类型一致的的Bean，在这种情况下，Spring将无法判定哪个Bean最适合该属性，所以不能执行自动装配。

byname（根据名称自动装配）：必须将目标Bean的名称和属性名设置为完全相同。

Constructor(通过构造器自动转配)：当Bean中存在多个构造器时，此种自动装配方式将会很复杂，不推荐使用。

下面看如何配置

package logan.spring.study.autowire;

public class Person {
    
    private String name;
    
    private Address address;
    
    private Car car;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Address getAddress() {
        return address;
    }

    public void setAddress(Address address) {
        this.address = address;
    }

    public Car getCar() {
        return car;
    }

    public void setCar(Car car) {
        this.car = car;
    }

    @Override
    public String toString() {
        return "Person [name=" + name + ", address=" + address + ", car=" + car + "]";
    }
}
package logan.spring.study.autowire;

public class Address {
    
    private String city;
    
    private String street;

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    public String getStreet() {
        return street;
    }

    public void setStreet(String street) {
        this.street = street;
    }

    @Override
    public String toString() {
        return "Address [city=" + city + ", street=" + street + "]";
    }
    
    

}
package logan.spring.study.autowire;

public class Car {
    
    private String brand;
    private String price;
    public String getBrand() {
        return brand;
    }
    public void setBrand(String brand) {
        this.brand = brand;
    }
    public String getPrice() {
        return price;
    }
    public void setPrice(String price) {
        this.price = price;
    }
    @Override
    public String toString() {
        return "Car [brand=" + brand + ", price=" + price + "]";
    }
    
    

}
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    
    <bean id="address" class="logan.spring.study.autowire.Address"
    p:city="Beijing" p:street="HuiLongGuan"></bean>
    
    <bean id="car" class="logan.spring.study.autowire.Car"
    p:brand="Audi" p:price="300000"></bean>
    
    <bean id="person" class="logan.spring.study.autowire.Person" 
    p:name="Tom" autowire="byName"></bean>


</beans>
package logan.spring.study.autowire;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Main {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        
        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans-autowire.xml");
        Person person = (Person) ctx.getBean("person");
        System.out.println(person);

    }

}

下面是输出结果

五月 20, 2017 3:41:56 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org[email protected]3eb07fd3: startup date [Sat May 20 15:41:56 CST 2017]; root of context hierarchy
五月 20, 2017 3:41:56 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [beans-autowire.xml]
Person [name=Tom, address=Address [city=Beijing, street=HuiLongGuan], car=Car [brand=Audi, price=300000]]

如果xml文件改成如下

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    
    <bean id="address2" class="logan.spring.study.autowire.Address"
    p:city="Beijing" p:street="HuiLongGuan"></bean>
    
    <bean id="car" class="logan.spring.study.autowire.Car"
    p:brand="Audi" p:price="300000"></bean>
    
    <bean id="person" class="logan.spring.study.autowire.Person" 
    p:name="Tom" autowire="byName"></bean>


</beans>

输出结果为

五月 20, 2017 3:43:15 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org[email protected]3eb07fd3: startup date [Sat May 20 15:43:15 CST 2017]; root of context hierarchy
五月 20, 2017 3:43:15 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [beans-autowire.xml]
Person [name=Tom, address=null, car=Car [brand=Audi, price=300000]]

可见这是根据名字来自动装配，如果地址address的id不是address就没有办法装配。

还可以根据类型来装配，如下：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    
    <bean id="address2" class="logan.spring.study.autowire.Address"
    p:city="Beijing" p:street="HuiLongGuan"></bean>
    
    <bean id="car" class="logan.spring.study.autowire.Car"
    p:brand="Audi" p:price="300000"></bean>
    
    <bean id="person" class="logan.spring.study.autowire.Person" 
    p:name="Tom" autowire="byType"></bean>


</beans>

输出结果：

五月 20, 2017 3:47:08 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org[email protected]3eb07fd3: startup date [Sat May 20 15:47:08 CST 2017]; root of context hierarchy
五月 20, 2017 3:47:08 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [beans-autowire.xml]
Person [name=Tom, address=Address [city=Beijing, street=HuiLongGuan], car=Car [brand=Audi, price=300000]]

但是根据类型装配有的时候会有问题，例如在xml里面配置两个相同类型的bean。

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    
    <bean id="address2" class="logan.spring.study.autowire.Address"
    p:city="Beijing" p:street="HuiLongGuan"></bean>
    
    <bean id="address3" class="logan.spring.study.autowire.Address"
    p:city="Beijing" p:street="HuiLongGuan"></bean>
    
    <bean id="car" class="logan.spring.study.autowire.Car"
    p:brand="Audi" p:price="300000"></bean>
    
    <bean id="person" class="logan.spring.study.autowire.Person" 
    p:name="Tom" autowire="byType"></bean>


</beans>

报错如下：

五月 20, 2017 3:48:29 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org[email protected]3eb07fd3: startup date [Sat May 20 15:48:29 CST 2017]; root of context hierarchy
五月 20, 2017 3:48:29 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [beans-autowire.xml]
五月 20, 2017 3:48:29 下午 org.springframework.context.support.ClassPathXmlApplicationContext refresh
警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name ‘person‘ defined in class path resource [beans-autowire.xml]: Unsatisfied dependency expressed through bean property ‘address‘; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type ‘logan.spring.study.autowire.Address‘ available: expected single matching bean but found 2: address2,address3
Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name ‘person‘ defined in class path resource [beans-autowire.xml]: Unsatisfied dependency expressed through bean property ‘address‘; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type ‘logan.spring.study.autowire.Address‘ available: expected single matching bean but found 2: address2,address3
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1357)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1249)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:553)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:866)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:542)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
    at logan.spring.study.autowire.Main.main(Main.java:11)
Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type ‘logan.spring.study.autowire.Address‘ available: expected single matching bean but found 2: address2,address3
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveNotUnique(DependencyDescriptor.java:173)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1116)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1066)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1342)
    ... 13 more

XML配置里的Bean自动装配的缺点：

在Bean配置文件里配置autowire属性进行自动装配将会装配Bean的所有属性，然而，若只希望装配个别属性时，autowire就不够灵活了。

autowire属性要么根据类型自动装配，要么根据名称自动装配。不能两者兼而有之。

一般情况下，在实际的项目中很少使用自动装配，因为和自动装配功能所带来的好处比起来，明确清晰的配置文档更有说服力一些。

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